Four criteria were used to select
the studies for inclusion in the meta-analysis: subjects 24
months old or younger, random assignment to a treatment (i.e.,
supplementary feeding) or a control group of subjects or
populations, experimental treatment restricted to the
administration of a nutritional supplement, and published data
allowing between-group statistical comparisons. Six studies met
these criteria. They were carried out in Bogota, Colombia [13];
eastern Guatemala [10]; West Java, Indonesia [11]; Kingston,
Jamaica [12]; Sui Lin, Taiwan [14]; and the Harlem district of
New York City, USA [15].

Research designs

Randomization

Five of the studies were
double-blind, randomized clinical trials (table 1). The sixth, in
Guatemala, had random assignment of villages, rather than
individuals, to one of two nutritional treatments.

TABLE 1. Research designs and
statistical methods used in data analysis in the six studies
analysed

a These numbers do not
represent the total number of subjects in the original study
but are restricted to those included in the analyses done
here. See reference for complete documentation.

The studies in Colombia, Jamaica,
and New York also included groups that combined educational and
nutritional interventions (i.e., educational stimulation). Those
groups were excluded from the present analysis.

Sample selection

The children in Colombia,
Jamaica, Taiwan, and New York were selected by age and dietary or
anthropometric criteria (table 2). The only criterion used in
Guatemala and Indonesia was age; however, pregnant women also
participated in the feeding programme.

TABLE 2. Characteristics of
populations and subjects in the six studies

In Colombia, Guatemala,
Indonesia, Taiwan, and New York, nutritional risk was defined by
a low protein and energy intake, whereas in Jamaica the criterion
for inclusion was anthropometry (< 2 SD below the reference
norm from the US National Center for Health Statistics).

For the purposes of the present
analyses, the subjects in the six studies were classified into
two groups according to age: infants and children 8-15 months
old, and children 18-24 months old. The age of the subjects in
Indonesia ranged from 6 to 20 months, with the mean ages of the
experimental and control subjects being 12 and 13 months
respectively [11]. As the mean age for all subjects (12 months)
fell within the range of 8-15 months, the subjects in the
Indonesia study were included in the younger group.

The meta-analysis for the younger
group covered all six studies, while that for the older group was
restricted to the studies in Colombia, Guatemala, and Jamaica.

Experimental intervention

The goal of the supplementary
feeding in all the studies except that in Indonesia was to fill
the subjects' energy and protein gap; however, the nutrient
composition and volume of the supplements varied among the
studies (table 3). The supplement in Indonesia was primarily
characterized by its high energy (approximately 400 kcal per day)
and low protein content. The role of micro-nutrients was not
assessed in any of the studies; however, the experimental and
control groups in Guatemala, Taiwan, and New York received
equivalent amounts of vitamins and minerals per unit of volume.

TABLE 3. Recipients and
composition of the nutritional supplements

Bogotá, Colombia

Entire family from last trimester of pregnancy
until child is 36 months old

The supplement was restricted to
mothers in two of the studies: in New York it was administered
only during pregnancy, and in Taiwan it was administered during
pregnancy and lactation. Supplements were also provided to
mothers during pregnancy and lactation in Colombia and Guatemala,
but infants and young children also received them. In Indonesia
and Jamaica the treatment was restricted to infants.

Developmental variables

The Bayley scales of mental and
motor development were used in Indonesia, Taiwan, and New York.
Two studies (Colombia and Jamaica) used the Griffiths scale. A
new scale was constructed for the specific purposes of the
Guatemala study, based on the Bayley and Gesell scales;
psychometric data on this custom-tailored scale are published
elsewhere [16].

Data analysis

The Rosenthal approach [17] was
used to determine whether the studies included in the two age
groups analysed tested a similar hypothesis. A test for
homogeneity of effect sizes followed the same approach.

Data were analysed using weighted
methods because of the wide range in the size of the samples.
Results on the developmental outcomes from the different studies
were combined according to the method proposed by Mosteller and
Bush [18]. Effect sizes were calculated according to the
procedures of Hedges [19], which weigh each study by the number
of subjects.

The partial correlation
coefficient between the intervention and each test was derived
from the corresponding effect-size value and was used as a
measure of the success rate as proposed by Rosenthal [17].

The hypothesis of homogeneity
cannot be rejected for either age group regarding the mental and
motor tests (table 4). Accordingly, the studies were pooled to
test the hypothesis that the early supplementary feeding of
energy and protein had beneficial effects on performance on the
developmental scales.

For the children 8-15 months old,
four of the six studies showed beneficial effects of the
supplementary feeding on motor development; none showed effects
on mental development (table 5). For the children 18-24 months
old, all three studies found significant effects of the
supplement on motor development; one showed effects on mental
development (table 6).

TABLE 5. Results of six studies
used to calculate combined motor score and combined mental score
at 8-15 months

With account taken of the
different numbers of subjects in each study, the supplementary
feeding had a significant effect on motor development scale in
both age groups. Similar statistics on mental development data
showed that the effect was restricted to the children 18-24
months old.

The partial correlation
coefficients derived from the combined effect-size values
indicate that the supplementary feeding improved motor and mental
tests by 9% and 2% respectively at 8-15 months. At 1824 months
the increases were 19% and 9% respectively.